Conventional self-contained power systems for vehicular and household applications typically use a battery pack for storing energy provided either by an intermittently available commercial utility or by independent energy sources such as solar panels. One problem with such systems is that the battery pack must be large enough to be able to provide at any time the maximum current expected to be drawn by the load. Considering the weight, expense and limited life of batteries, the conventional approach is often an unsatisfactory solution in situations where high output current is needed only for relatively short periods of time in the course of a day.
Electric power storage systems which use capacitors instead of batteries as the storage elements are not new. They have, however, found little practical application in high-power environments for a number of reasons. For one, the amount of energy storable in a battery was substantially larger than that storable in a capacitor; and for another, much of the energy stored in a capacitor was unusable because in a commercial power environment, the output voltage of a capacitor dropped to inadequate levels when only a relatively small portion of its stored energy had been expended.
The first of these problems has recently been resolved by the introduction of new types of capacitors termed supercapacitors or ultracapacitors which have a vastly increased energy storage capability without a corresponding increase in size. Nevertheless, storage systems based on these capacitors would still require relatively frequent recharging when charged from, e.g., a 440-volt supply and used to power a 220-volt device. Raising the charging voltage would be impractical because this would require additional insulation in the capacitors and make them bulky; and the output voltage cannot be changed because it is dictated by the nature of the load.